This invention relates to the field of optical recording and, more particularly, to improvements in optical heads for writing and/or reading information on a radiation-sensitive recording element, such as an optical disk. More specifically, this invention relates to improvements in optical read/write heads of the type which employ integrated optics.
In recent years, considerable effort has been expended in miniaturizing the electro-optical systems used to write and/or read information on optical recording elements. Such systems, often referred to as optical "heads" or "pick-ups," typically comprise at least one diode laser, various bulk optical elements (e.g. prisms, lenses, mirrors, etc.) for expanding, shaping, deflecting and focusing the beam emitted by such laser, and a plurality of individual detectors which serve to detect from the laser energy reflected (or transmitted) by a recording element the recorded information. Such detectors also serve to develop focus and tracking errors signals for controlling the focus and tracking position of the laser beam on the recording element. With the advent of the diode laser, such electro-optic read/write systems have been reduced in size to a volume of less than about 15 cubic centimeters (i.e. one cubic inch). Equally important is the fact that the mass of such electro-optic systems has been reduced to less than about 100 grams.
Notwithstanding significant strides in miniaturizing optical heads in optical recording systems, the size and weight of such elements can still have a limiting effect on the time required to access information on the recording element. Even miniaturized optical heads of the above type are sufficiently massive as to require significant time to settle into a read/write position after a "track jump" in which the head is rapidly moved relative to the recording element to access information at a different location.
In more recent years, the attention of some researchers has been focused on the field of integrated optics in an effort to further miniaturize optical read/write heads. European Patent Application No. 174,008, for example, discloses a variety of integrated optical heads, each comprising an optical waveguide into which diode laser energy is coupled. Optically integrated in the waveguide is a lens (e.g. a Fresnel lens, Bragg grating lens or Luneburg lens) for collimating the light incident thereon from the laser source, and a focusing grating coupler (FGC) for deflecting incident collimated light out of the waveguide and for bringing such light to a focused spot on the surface of the recording element. Such integrated optical heads also include a detector package for sensing light reflected obliquely from the recording element so as to detect the recorded information and the focus and tracking accuracy of the incident light. The respective outputs of such detectors are used to develop focus and tracking error signals which, in turn, are used to maintain the spot in precise focus on the recording element and to control the position of the spot relative to a desired track of information.
In one embodiment of the above-noted application, the waveguide is made of a material (e.g. LiNbO.sub.3 or PLZT) which exhibits the well known electrooptic effect, and the focus and tracking error signals are used to selectively vary the refractive index of a portion of the waveguide which defines a control lens used to control the focus and tracking position of the read/write spot. Such variation is achieved by a complex array of closely spaced electrodes arranged on the top surface of the waveguide. When coupled to a voltage source, these electrodes produce fringing fields which alter the refractive index and shape of the integrated lens, thereby changing its focal length and causing the focused spot to move in a direction perpendicular to the optical axis.
While integrated optical read/write heads such as disclosed in the above-referenced European application are considerably less massive and more compact vis-a-vis optical heads employing discrete bulk optical elements, such integrated heads, and in particular those that incorporate several sets of electrodes on the waveguide for varying the tracking position of the focused spot, are relatively complicated in construction and in the manner in which they are addressed. Moreover, because some of the electrode pairs are spaced relatively far apart (of the order of millimeters) on the surface of the waveguide, relatively large voltages (e.g. thousands of volts) need be applied between the electrodes in order to produce the fringing fields required to sufficiently alter the refractive index of the waveguide to achieve the desired effect.